This application pertains to the art of electrographic recording systems and more particularly to an apparatus and method for forming electrostatic latent images on a record medium in accordance with information provided by electronic signals.
This invention is particularly applicable to high speed recorders such as peripheral equipment for computers, telecopy, and the like and will be described with particular reference thereto. It will be appreciated that the invention has broader aspects for recording alpha-numeric, pictorial, and graphic data. It is amenable to receiving electronic signals in a raster format as would be used from producing a CRT or video display.
The electrographic recording process is generally characterized by two steps. The first step is the establishment of an electrostatic latent image on a record medium by electrically charging selected areas of the medium with electrostatic recording electrodes connected to charging circuit means. The second step is rendering the electrostatic latent image visible by toning or developing the charged areas on the record medium.
More specifically a special paper is passed over an image head which contains the electrostatic recording electrodes. A suitable paper for use as the record medium is described in U.S. Pat. No. 3,657,005 assigned to the same assignee as the present application. Generally, the image head consists of a generally linear assembly of needle-like recording electrodes or styli. Adjacent the assembly of recording electrodes is one or more assemblies of complementary electrodes or shoes. The latent image is formed on the record medium by applying a negative voltage to the recording electrodes and a positive voltage to the complementary electrodes. A potential difference between the recording and complementary electrodes of approximately 500 volts results in the deposit of a negative electrostatic charge on the record medium under a recording electrode. Suitable image heads are disclosed in U.S. Pat. Nos. 3,611,419 and 3,653,065 assigned to the same assignee as the present application.
Generally, the recording electrodes are charged with a voltage of a first polarity, for example -300 volts, and the complementary electrodes are charged with a voltage of the opposite polarity, for example +300 volts. When these two voltages are applied contemporaneously to a recording electrode and complementary electrode, a localized negative charge is deposited on a dielectric surface of the above special paper. If the two voltages are not applied contemporaneously the potential fails to achieve the amplitude required to deposit a charge on the above paper.
The principal that a recording electrode and adjacent complementary electrode must be actuated contemporaneously to establish a latent image allows for a reduction in the number of circuits necessary for actuating the recording and complementary electrodes. As shown in more detail, in U.S. Pat. No. 3,653,065, supra, the assembly of recording electrodes are divided into a number of arrays and the arrays divided into two groups of alternating arrays. In each group, like-numbered electrodes in each array are connected together. A plurality of complementary electrodes are similarly provided, specific complementary electrode(s) are mounted adjacent each array of recording electrodes. To form a latent image under one or more of the recording electrodes in a selected array, the selected recording electrodes of one group and the adjacent complementary electrodes to the selected array are actuated in coincidence. Actuating the selected recording electrode actuates like-numbered recording electrodes in every array in the group. However, if only the complementary electrodes adjacent to the selected array are actuated simultaneously, then a latent image is formed only under the selected recording electrodes.
To print a line of data, i.e. a set of electronic signals indicating whether an electrostatic charge is to be or not be deposited under each recording electrode, the system divides the line into segments. Each segment corresponds to one of the arrays of recording electrode. The segments corresponding to the arrays are serially connected alternately to the first and second groups of arrays. Contemporaneously with the segment corresponding to the first array being connected thereto, the complementary electrodes adjacent the first array are actuated. The process continues similarly for the second, third, fourth, and subsequent arrays until the entire line of data has been recorded. The record medium advances slightly and the process is repeated for second and subsequent lines of data.
After the record medium has received the latent image, it advances to a development area in which toner is supplied to the surface. The toner includes black particles which adhere only to the charged areas of the surface. The excess toner is removed from the non-charged areas of the record medium and the toner fixed to the charged surface areas. The record medium then emerges from the recorder as a permanent, printed record.
One of the principal problems with this type of recorder is what is known as a striping effect. That is, the intensity of the toned electrostatic image produced by the recorder varies in a repeating pattern across the image head. The toned images vary from grey to black with a periodicity comparable to the array sizes. This resultant striping is undesirable.